1. Field of the Invention
The invention relates to a block for partially limiting a radiation beam, providing this limitation with a reduced penumbra; it also relates to a space-saving collimator equipped with such blocks. Collimators are widely used in the radiotherapy field for example, where they allow the geometry of a useful beam to be defined with respect to a desired treatment area.
2. Description of the Prior Art
These collimators are generally formed by a first and second assembly for limiting the radiation beam, superimposed in the path of this latter and acting on two axes orthogonal and transversal to a reference direction of this beam, so as to delimit the useful beam. This first and second limiting assembly each comprise a first and a second movable partial limiting block, one end of which is intended to cut off a part of the beam; the spacing apart between these ends forming a slit into which passes the radiation beam. This beam is thus partially limited by passing through a first slit formed by the first limiting assembly and totally limited by passing through a second slit, formed by the second limiting assembly, at the output of which it forms the useful beam.
The position occupied by these first and second limiting devices, with respect to a reference direction of the radiation beam, defines a useful beam centered or off-centered with respect to this reference direction.
An important quality of the useful beam resides in its delimitation with a minimum penumbra; to this end, the ends intended to cut off the beam have a not inconsiderable thickness, which forms a flat surface called active surface in the description which follows. The orientation of this active surface, with respect to a source emitting the beam, is of great importance for obtaining the useful beam with a minimum penumbra.
This means that the orientation of the active surfaces may be modified for maintaining this orientation for each modification of the useful beam.
In the prior art, this condition is fulfilled by means of mechanical means and drive means whose construction is difficult and complicated. Thus, for example, a desired useful beam is obtained by a movement of the first and second limiting blocks such that their movement towards or away from each other is accompanied by a rocking movement of their longitudinal axis, so as to obtain correct orientation of the active surface which they comprise.
Taking into account a not inconsiderable length of these first and second limiting devices, the space required for allowing this rocking is large. A collimator thus formed has a volume which adversely affects the use thereof; furthermore, its assembly which is time-consuming and difficult requires costly machining operations, because of the complication of the mechanical and drive means used.
The present invention relates to a block for partially limiting a radiation beam, simple to mount and limiting a beam with a minimum penumbra because of its design which does not require the orientation of the active surface for each new position occupied by this block. It also relates to a collimator comprising such blocks in accordance with the invention, for determining the useful beam; a collimator in accordance with the invention takes up less space and is more simply assembled with respect to collimators of the prior art, its mechanical and drive means being simplified with respect to these latter.